A frequency selective heteronuclear polarization transfer technique is introduced for rotating solids. In this method, weak or medium strength radio frequency (rf) fields are applied to establish Hartmann-Hahn cross polarization that explicitly depends on the resonance offset of the involved nuclei. Under these conditions. SPECtrally Induced Filtering In Combination with Cross Polarization (SPECIFIC CP) can be achieved for spectral simplification or assignment purposes in heteronuclear spin pairs. Frequency selective transfer occurs if a broadband, adiabatic CP from protons to carbons/nitrogens is followed by a second CP step from '3C to '5N or 5N to '3C that selects the '5N, '3C pair of interest. To further enhance the transfer efficiency, an adiabatic transfer profile can be combined with the SPECIFIC CP approach without losing the offset selectivity of the transfer. The experiment is very forgiving with respect to the size and the homogeneity of the applied rf fields. Usually, transfer efficiencies of 40-60% of the direct CP (1H->13C/15N) are obtained at 200-500 MHz fields. We have applied the SPECIFIC CP technique to establish dipolar filtering or two-dimensional correlation useful for backbone of side-chain assignment in peptides (N-acetyl-valine, N-acetyl-arginine dihydrate and formyl-MLF). Since SPECIFIC CP can create zero- or double quantum coherence between isolated NH-CO, NH-CU backbone pairs, this technique can also be applied to determine local backbone torsion angles in the solid state. For spectral assignments, an alternative correlation experiment (SPECIFIC HETCOR) may be useful when only particular spectral ranges in the indirectly detected dimension are desired. In this case. the carrier frequency can be stepped through the offset of interest thereby reducing the total acquisition time. Our results for formyl-MLF indicate that the transfer efficiency of the SPECIFIC CP is sufficient for the investigations of 15N labeled polypeptides. The high transfer efficiency also permits the investigations of larger biological systems, and we have successfully applied SPECIFIC transfer steps to study the Ala8l-Arg82 peptide bond in the 26 kDa trans-membrane protein bacteriorhodopsin. Since only one of the seven arginine residues is directly bonded to alanine, 13C labeling of the carbonyls of all 29 alanine residues and 15N labeling of the amino groups of all 7 arginine residues introduces a unique '3C, 15N pair. The SPECIFIC CP experiment shows disorder in the Ala8l-Arg82 peptide bond that is obscured in the non-selective CP experiment. This disorder varies among states of the protein, although the chemical shift changes are small.